The present invention relates to a semiconductor memory device, and, more particularly, to a static RAM (i.e, Random Access Memory), which is composed of MIS (i.e., Metal Insulator Semiconductor) type field effect transistors (which will be shortly referred to as "MISFETs") and a fabrication process thereof.
In an MIS type semiconductor integrated circuit device (which will be shortly referred to as an "IC"), and input protection circuit composed of a resistor and a diode may be connected between an input terminal and a gate so as to protect the gate of an MISFET which is to be fed with a signal coming from the outside of the IC. It is conceivable to use a diffusion resistor, which is prepared in a semiconductor wafer by the diffusion technique, as that input protection resistor. According to our investigations, however, it has been found that, in case a C-MOS is used in the peripheral circuit unit other than the memory cells of the aforementioned static RAM, such diffusion resistor never fails to be formed with a PN junction so that the ratch-up phenomenon takes place. In order to prevent this phenomenon, we have conceived to use a polycrystalline silicon film (which will be shortly referred to as a "poly Si film"), which is formed over the field oxide film of the semiconductor wafer, as the input protection resistor. As a forming method of this input protection resistor, we have also conceived to use the poly-Si film which is formed simultaneously with the poly-Si gate electrodes of the MISFETs of the peripheral circuit unit, the poly-Si gate electrodes of the MISFETs in the memory cells and the poly-Si word lines in the memory cells. In this case, however, in order to shorten the propagation delay time of the signals at the gates and the word lines, the aforementioned poly-Si film has to be doped with phosphorous to have its specific resistance lowered (to 30 .OMEGA./.quadrature., for example). In order to have the desired resistivity (about 2 K.OMEGA.) required of the input protection resistor, therefore, it is necessary to enlarge the wiring length of the poly-Si film. As a result, it has also been found that the area occupied by the protection resistor is enlarged to augment the chip size, which is disadvantageous for the improvement of integration.